The warships left Port Sudan on the Red Sea earlier, after a visit that coincided with Khartoum's denial that Iran is making weapons in Sudan.

Acting U.S. State Department spokesman Mark Toner was quoted by AFP as having said the United States was "obviously watching that closely."

"We monitor Iran's activities in the region very closely," he added.

Sudan's links to Iran have come under scrutiny after Khartoum accused Israel of sending four radar-evading aircraft to strike the Yarmouk military factory in the heart of the capital Khartoum at midnight on October 23.

The United States was "aware of an explosion," Toner said, but added that Washington had no more details of what the incident entailed and how it was caused.

Turning to the visiting Iranian warships, he added, "It's hard for us to know what the details are of this visit right now... Certainly we would be concerned, but we don't have any more details."

Sudan's army spokesman Sawarmi Khaled Saad told the official SUNA news agency on Tuesday that the Iranian ship visit "will support strong political, security and diplomatic relations between the two states."

Saad said the port call was a chance for Sudanese naval personnel "to see advanced weapons and advanced ships."

Earlier, a member of the Iranian parliament confirmed that Tehran had a close relationship with Khartoum, transferring manpower, technology and materials to Sudan. However, the official said, no Iranians were present in the arms factory that was mysteriously blown up last week.